Giant Geyser truly lives up to its name. It is currently the second tallest geyser in the world, only Steamboat Geyser located in Yellowstone's Norris Geyser Basin is taller. An eruption of Giant can reach 250 feet, last over an hour and put out an estimated one million gallons of water. For comparison, a large Old Faithful eruption reaches about 150 feet, lasts less than 5 minutes (the biggest part lasts less than one minute) and puts out around 10,000 gallons of water.

Giant is a cone-type geyser. Once it starts, it puts out a steady and massive jet of water that lasts until almost the end on the eruption. The start of Giant is very impressive. The water column quickly rises, in one steady motion, to its maximum height. The maximum height is only held for a few minutes before it slowly begins to shorten but even so, the water column is often still near 100 feet tall 30 minutes into the eruption. The maximum height of the eruption can vary greatly, form 150 feet to over 250 feet. No matter what the height though, the eruption lasts an hour or more and puts out an impressive amount of water. Thus, even a "short" height eruption is very spectacular.

Types of eruptions:

In the early 1950's, the last time Giant had major activity, Naturalist George Marler categorized two types of eruptions, Normal Function and Mastiff Function. While some of the eruptions seen in the 1980s, early 1990s and during the increased activity since 1996 did not fall neatly into Marler's two categories, but instead fell somewhat in-between, it is still worthwhile to know the general characteristics of the two functions.

Normal Function:
In a normal function eruption, only Giant erupts. Marler noted that these eruptions were usually shorter, approximately 150 feet. (It should be noted that one of the tallest eruptions during 1997, about 230 feet, was a normal function eruption.) Marler also noted that the hot period prior to Giants eruption was dominated by the platform vents on the southwest side (the Giant side) of Giant's Platform.
Mastiff Function:
In a Mastiff function eruption, Mastiff Geyser and sometimes Catfish and Bijou Geysers also join the eruption. Mastiff Function eruptions are usually among the tallest Giant eruptions, sometimes reaching over 250 feet. Marler noted that the hot periods prior to these eruptions were dominated by the platform vents in front of Giant and by surging from Mastiff. A Mastiff Function eruptions starts with an eruption of Mastiff Geyser. Mastiff can reportedly reach 100 feet in one of these eruptions but in eruptions seen since 1996 the maximum was closer to 50 feet. Giant's eruption can start during Mastiff's eruption or just as Mastiff is dying down. Marler reported that if Giant started during Mastiff, then Giant would reach about the same height as Mastiff and hold for a short time before taking over the eruption. If Giant starts while Mastiff is dying down, then Giant will rocket to its full height in one uninterrupted motion. Either way, it is very impressive to see two large geysers erupting simultaneously so close together. During a Mastiff Function eruption, Catfish Geyser can also join the eruption and erupt to an abnormal height. Catfish can send up a slender plume to 75 feet. Even a 40 foot tall eruption is spectacular. Catfish's eruption occurs near the time of the start of Giant. Thus, in a full blown Mastiff Function eruption, Giant, Mastiff and Catfish are all erupting simultaneously. During all of this, Bijou may be roaring in a loud steam phase. The only problem with this spectacular scene is that all three geysers are so close together, less than 75 feet apart, that the smaller two, Catfish and Mastiff often become lost in the huge water column of Giant.
Connections to other geysers:

Giant geyser is connected to most of the geysers and pools in its area. It is obviously connected to the geysers on its platform: Bijou Geyser, Catfish Geyser, Mastiff Geyser, the Platform Vents and Turtle Geyser. It is also connected to the Grotto Group, Oblong Geyser and the Purple Pools. It is possibly connected to Daisy Geyser.

What to look for:

Ultimately, what you are looking for is a hot period. All Giant eruptions start with a hot period but not all hot periods result in an eruption of Giant. A hot period is characterized by the eruption of at least one of the platform vents and high water in Mastiff's front vent. Hot periods can range in strength from one that only involves one platform vent to one that includes eruptions from most of the eleven or so platform vents along with heavy overflow from Mastiff and surging from Mastiff to as much as 10 feet. These strong hot periods really get your heart pumping but unfortunately, even these have been known to sometimes end without an eruption of Giant.

The hot period is caused by the rising of water in the system. Because of the necessity to stay on the boardwalk, the first manifestation of this rise in water that can be seen is that Bijou Geyser stops its almost incessant play. If a hot period follows this Bijou pause then after a minute or so, a rising pool of water should become visible in Mastiff's front vent. As this approaches overflow, water may also start to overflow from some of the platform vents. Eventually, some of the platform vents will start erupting. this is the start of the hot period. The hot period can stop at any time. If the water in Mastiff drops and Bijou restarts then the hot period is probably dying and Giant will not erupt. As long as Mastiff stays up, or only drops and rebounds, then, even if Bijou restarts, there is still a chance that Giant could erupt.

If the hot period continues, it usually will get stronger. Mastiff can have heavy overflow and strong surging. More of the platform vents can start erupting. Eventually, Giant will start surging. These surges will become straighter and taller and can fill the entire cone and spill water out the front of the cone. Even at this point, the hot period can stop and Giant not erupt but this is fairly rare. If the activity continues, at this point one of two things can happen. If it is a Mastiff Function, Mastiff Geyser will start erupting in a massive two pronged eruption to 50 feet or more. Then a few minutes later Giant will join in. If this is a Normal Function eruption then, Giant will rocket skyward and the eruption is on.

When to expect a HOT PERIOD

While Hot Periods can occur at almost any time, they are often closely tied to what Grotto Geyser is doing. Hot Periods that lead to an eruption of Giant often occur near the start of any Grotto eruption or 4-5 hours after the start or 4-6 hours after the end of a long mode eruption of Grotto (a Grotto Marathon eruption).

Some of the temperature
data used to derive the eruption times and durations used in this section were
collected by Ralph Taylor under a National Park Service research permit, and the
remainder was collected by personnel working for the Geology Department of the
Yellowstone Center for Resources (including Ralph Taylor). The loggers are a
combination of loggers owned by the NPS and Ralph Taylor. Analysis of the raw
temperature data to extract the eruption data was performed by Ralph Taylor. The
eruption time files on this website may be used provided that Yellowstone
National Park is credited for the temperature data and Ralph Taylor is credited
for the eruption times.

BIG NEWS -- Giant eruption 7/7/2017!

More details as they come in; first eruption in a VERY long time!

Followup: Also an apparent eruption 10/9/17! Not seen in person, but steam cloud noted on the webcam and signs on the platform were shifted in ways consistent with an eruption.

This picture of Giant's cone has been attributed to William Henry Jackson. His first trip to Yellowstone was as photographer for the 1871 Hayden expedition. His photographs were instrumental in persuading Congress to set aside Yellowstone as a National Park.

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for more information. Last update 01-29-2017